Proper development of all multi-cellular organisms requires not only correct spatial control of cellular interactions, but also correct timing of specific gene expression programs during development. Altering the timing of many developmental processes can significantly affect an animals final body size, reproductive ability and ageing. In insects, timing of developmental transitions, i.e. molting and metamorphosis, is mediated by 20-hydroxyecdysone (20E), a small circulating lipophilic steroid hormone that binds to and activates transcription factors of the nuclear receptor superfamily. Although the genetic hierarchy that controls responses to 20E has received considerable attention, little is presently known about mechanisms that control the timing of hormone production and its release. In this proposal we will determine how prothoraciotropic hormone, a peptide released from neurons that innervate the prothoracic gland (PGs) control hormone production and release. In aim 1 we will determine when the PG cells receive PTTH signals and what regulates their competence to do so. In aim 2 we will characterize additional upstream and downstream inputs that regulate PTTH signaling through the receptor tyrosine kinase Torso. Of particular interest is to determine if the PTTH ligand processing is required for PTTH signaling and if PKC, PLC and the IP3 receptor, contribute to PTTH signaling in Drosophila and how their activity is coupled to Torso activation. In aim 3, we will identify key transcriptional control elements in the regulatory regions of the ecdysone biosynthetic enzymes phm, dib and spookier, in order to learn how PTTH signals regulate ecdysone production. In aim4 we will use genetic studies and imaging methods to test the hypothesis that, contrary to present beliefs, release of ecdysone from the prothoracic gland during the metamorphic molt is a vesicle mediated process stimulated by PTTH. Impact on human health: In humans, a major developmental transition involving steroid hormones takes place as adolescents acquire sexual maturity. This transition is also controlled by a neuropeptide signal in conjunction with nutritional and metabolic cues, although little is presently known about how these signals are integrated to time the transition appropriately. The studies described here will help identify basic molecular programs that control developmental transitions and thereby provide a paradigm for understanding how these processes might be regulated in vertebrates including humans. In addition these studies will provide a new understanding of how steroid hormones are released from endocrine cells. PUBLIC HEALTH RELEVANCE: The studies described here will help identify basic molecular programs that regulate steroid hormone production and secretion in insects. Since related developmental transitions and steroid release mechanisms occur in many organisms, including humans, this work will provide a paradigm for understanding how different steroidogenic regulatory cues are integrated by the neuroendocrine circuit to control development.